Ship overcurrent relays are used to protect sensitive equipment against over or under current conditions. By using current transformers (CTs), these protective relays monitor large AC currents common to large motor starters, circuit breakers, and transformers.
Ship current transformers CT which steps down the monitored current to a secondary (output) range of 0 to 5 amps (AC) to power the protective relay.
If the monitored current becomes excessive, the relay triggers a contact-switching auxiliary control circuitry to the breaker’s trip coil.
By using the selected time delay between 0.05 and 30s, short current peaks can be bridged.
Connecting the ship current transformer CT to the current monitoring relay terminals is depicted on the relay diagram’s symbol Terminals coded with the letter B represent connections to the current transformer.
Terminals B1 to B3 connect ship current transformer CTs with different step-down current ratios.
C shows another connection terminal, common for any CT ratio.
Example of a current monitoring relay and a related electrical diagram
The power is applied across A1/A2, and the current signal being monitored is applied across B1 and C. Depending on the measurement of the current ratio, the following connections are allowed:
B1/C: 3-30 mA, B2/C: 10-100 mA, B3/C: 0.1-1 A.
The relay operates with changeover contacts, the motor starter control circuits use NO (normally open) contacts in series with stop button contacts. The result Is that an occasional break in the power supply will stop the motor.
The relay can operate both contact groups, either with or without a time delay, depending on the rotary selector switch (Tv) position.
Whatever mode of operation is adjusted for the relay (either instant or delayed), only one relevant reference number will be used throughout a diagram.
Relay set for overcurrent detection (/>)
If the current Is greater than the set threshold, the output relay is energised, with or without a time delay. When the current returns to a value below the threshold, depending on the hysteresis setting, the relay Is Instantly de-energized.
How to set ship relay set for undercurrent detection
If the current is less than the setting threshold IS1, the output relay Is energized, with or without a time delay.
When the current returns to a value IS2 above the threshold, depending on the hysteresis setting, the relay is instantly de-energized.
